This invention relates generally to the provision of an independent scrollable display region (sometimes referred to as a "window") in a cathode-ray tube (CRT) display and to the provision of scrolling in the aforesaid window. More particularly, the present invention relates to a relatively simple circuit for providing both the windowing and the scrolling in a bit-mapped data display system.
Many schemes exist in the prior art for providing both a window function and a scrolling function U.S. Pat. No. 4,342,991 dated Aug. 3, 1982 by Richard N. Pope et al entitled "Partial Scrolling Video Generator" describes a scrolling system. Stated at the top of column 3 of this patent, the scrolling is accomplished by eliminating the refresh memory addressing from the CRTC (cathode-ray tube controller) of conventional design and substituting a presettable binary counter as a refresh address counter and an indirect address counter which alternatively access the address input of a refresh memory through a first multiplexer.
U.S. Pat. No. 4,284,988 dated Aug. 18, 1981 by Charles L. Seitz et al entitled "Control Means to Provide Slow Scrolling Positioning and Spacing in a Digital Video Display System" describes another means of providing a scrolling function. As described in column 11 of that patent, a character wheel is employed and three parameters (namely, the initial position, the count, and the mask) are loaded into respective registers at the beginning of each character line. By adjusting these parameters, the initial scan lines of the character line can be deleted with the remaining scan lines automatically moving up with additional scan lines being added at the bottom of the character space, and the character consequently appears to move up.
Another scheme is described in U.S. Pat. No. 4,375,638 dated Mar. 1, 1983 by David B. O'Keefe et al entitled "Scrolling Display Refresh Memory Address Generation Apparatus". As stated in column 2 of that patent, beginning at line 61, rows of character information are stored in fixed length rows in the display controller refresh memory. The display controller includes a roll register which is loaded with the value corresponding to the number of rows that the information on the display screen is to be rolled (scrolled) and this value is used in generating a relocated address which is used to retrieve the character information stored in the refresh memory.
Still other patents relating to this field include: U.S. Pat. No. 4,386,410 dated May 31, 1983 by Y.C. Pandya et al entitled "Display Controller For Multiple Scrolling Regions"; U.S. Pat. No. 4,412,294 dated Oct. 25, 1983 by L.F. Watts et al entitled "Display System With Multiple Scrolling Regions"; U.S. Pat. No. 4,141,003 dated Feb. 20, 1979 by L. Felsenstein entitled "Control Device For Video Display Module"; U.S. Pat. No. 3,903,510 dated Sept. 2, 1975 by G.C. Zobel entitled "Scrollin Circuit For a Visual Display Apparatus"; and U.S. Pat. No. 3,683,359 dated Aug. 8, 1972 by A.J. Kleinschnitz entitled "Video Display Terminal With Automatic Paging".